An ordinary sort of propeller-shaft for a small screw steamer is shown by Fig. 109. The number of pieces made use of in forging the shaft is about three ; but if the shaft is small, two are sufficient. One of these two becomes the shaft proper, and the other piece the disc, which is welded to one end in the manner described for an intermediate screw-shaft.

The diameter of the piece selected may be only equal to the forged diameter, if the iron is soft and fibrous; if not, the piece should be larger, that it may admit a welding and steam-hammering to produce a tenacious metal. The smith will observe whether a screw is to be formed at the propeller end; if so, he will be careful to make that part sufficiently solid.

Propeller-shafts may be made also by the drawing down of a lump whose sectional area is the same as that of the disc; the stem thus produced being intended for part of the shaft or axle. To this short portion two or three other pieces are welded to complete the work.

The two parts represented by G are linings of gun-metal, placed upon the shaft to receive the friction. When these finings are made by the metal being poured around in a liquid state, the shaft at those places needs no lathe-turning, being roughly forged to the finished diameter by the smith. After the gun-metal is poured, the shaft becomes bent, and is afterwards straightened in a lathe, either in the smithy or elsewhere.

The smith effects the straightening by heating the shaft and afterwards rotating it in a lathe, and marking the prominent sides at those places which rotated truly previous to the casting. These projecting sides are then put downwards, a few blocks are put upon the lathe-bed and beneath the bent portion of the shaft; wedges are then driven in between the blocks and the shaft, by which it is forced up and straightened while a few blows are given to the upper side. The amount of hammering needed is but small, if the shaft were heated to redness. During the whole of such straightening the work should be well supported with chains; it is also necessary that the centre recesses be large, and that the lathe poppet-screw be frequently screwed in, to fill the gap made by the continual shortening of the work while in the lathe.